Tip Jar on Bar???

So our bartender emailed me to see if it was ok to put a tip jar on the bar. My initial reaction is that it’s tacky, and don’t want my guests feeling g as though they need to tip. When I said we’d rather take care of the tip ourselves at the end of the night, she sent me a “tipping guide”. It says that the average tip for my wedding size (80 guests) is $40 per hr. We hired her for 7 hrs. This would mean she is expecting a tip of $280. Her fee is $300. Does she seriously expect A 100 % tip??!! Is this normal? I was thinking to tip her the standard 20% . Anyone else have this experience? Thoughts on the tip jar?